The present invention relates generally to smart probe apparatus for use with oscilloscopes (scopes), and more particularly to probe and a process for automatic self-adjustment of a scope""s bandwidth based on knowledge of the connected probe""s communicated bandwidth.
It is difficult to optimize a probe and scope to operate together. Often times a scope will be designed with a certain xe2x88x923 dB bandwidth and a probe is designed to be used with that scope. If the probe has the same xe2x88x923 dB bandwidth then when it is attached to the scope, the system including both the scope and an attached probe will have a bandwidth at the probe tip possibly quite a bit lower than either component separately.
This represents a problem for the user since the bandwidth to the probe tip is often not known. A common solution is to design the probe to be peaked such that it will be 0 dB at the scope""s bandwidth so when it is attached to the scope the whole system still has the same bandwidth as the scope by itself. Adjusting the probe""s transient and frequency response to be optimum with a particular scope may not well suit the probe to other oscilloscopesxe2x80x94particularly future models not yet designed. For example, plugging the probe into a future higher speed scope may result in an under-damped transient response since the probe was designed assuming the slower scope""s front-end to dampen the response. Another solution would be to under-damp the scope""s transient response so it has enough margin at its bandwidth to guarantee a particular system bandwidth with the probe attached. A problem then arises, however, that involves sacrificing settling time and noise performance when a particular probe is not being used.
This invention takes advantage of the fact that the front-end of an oscilloscope normally has excess HF adjustment range (or it can be designed to have such) that can be used to peak the frequency response of the scope automatically as needed to obtain optimum system response. It also makes use of probes/scopes utilizing the TekConnect Probe Interface available from Tektronix, Inc. of Beaverton, Oreg., assignee of the present invention. The TekConnect Probe Interface, in turn, utilizes the so-called I2C bus, familiarity with which is assumed.
The I2C bus is described and illustrated in U.S. Pat. No. 4,689,740 entitled TWO-WIRE BUS-SYSTEM COMPRISING A CLOCK WIRE AND A DATA WIRE FOR INTERCONNECTING A NUMBER OF STATIONS, issued Aug. 25, 1987 and assigned to U.S. Phillips Corporation. See also U.S. Pat. No. 4,672,306 entitled ELECTRONIC PROBE HAVING AUTOMATIC READOUT OF IDENTIFICATION AND STATUS, issued Jun. 9, 1987 and assigned in common with the present invention. These two patent disclosures are incorporated herein by this reference.
Briefly, the invention involves storing information about the probe frequency response in the probe itself. The information preferably consists of a set of one or more characteristic frequency response data points. Each data point includes at least the gain of the probe at a given frequency. More preferably, each data point includes the complex transfer characteristic of a probe (S21 in FIG. 2) and optionally the complex output reflectivity of the probe (S22 in FIG. 2) and the frequency (F in FIG. 2) at which the data applies. A variable number of frequency points can be stored, as necessary, to adequately describe the probe""s response. The probe also preferably stores a header containing the number of stored data points. The invention readily extends to probes having multiple gain settings or multiple bandwidth settings, by simply storing multiple sets of S21, S22 and F data.
When the probe is attached to a scope, the scope reads the frequency response information from the probe in accordance with any suitable communications format and protocol. Using information about its own frequency response and input voltage standing wave ratio (VSWR), the scope undertakes to peak its frequency response (by which is meant to add more gain at high frequency), thereby compensating for the frequency response of the probe.